The present invention relates to a signal conversion method, and more particularly to a method of converting digital signal to analog signal.
Conventional methods of converting digital signal to analog signal include the voltage (current) weighted method, the R-2R network method, and the pulse width modulation method.
Referring to FIG. 1, the voltage (current) weighted uses 8,4,2,1 binary weight ratios to achieve digital to analog conversion. According to this conversion method, we can obtain V.sub.O =V.sub.R.times.(A.times.2.sup.3 +B.times.2.sup.2 +C.times.2.sup.1 +D.times.2.sup.0). The circuit for this voltage (current) weighted method is complicated. When the number of bits to be computed and the number of outputs are increased, the number of operational circuits must be relatively increased by a factor of 2.sup.n. Therefore, the cost of this method is high. Because the difference between MSB (the most significant bit) and LSB (the least significant bit) is large, the total accuracy is difficult to control the tolerance of MSB usually affects the tolerance and linearity of LSB a lot.
Referring to FIG. 2, the R-2R network method is similar to the voltage (current) weighted method. The difference of in the element value, as of element between MSB and LSB in the converting circuit according to R-2R network method is less significant, and can be relatively easily controlled, however the elements required for the converting circuit are complicated.
Referring to FIGS. 3 and 4, PWM (the pulse width modulation method) improves the drawbacks of low accuracy and complicated circuit structure, however it needs to run complicated comparison and switching actions. Because the frequency of actions is increased by a factor 2.sup.n, the drawbacks of high noise, high power consumption, low speed and poor linearity become inevitable.